Various technologies for selectively adsorbing and separating carbon monoxide (CO) from a gas mixture have been researched ever since the fact that carbon monoxide can be absorbed using an ammonium chloride solution became known in the chemical technology field.
In order to more effectively separate carbon monoxide, various metals, such as nickel, cobalt, copper, silver, and the like, may be combined with solid materials. Among them, an adsorbent including copper compounds is known to most effectively adsorb carbon monoxide. That is, in industrial fields, carbon monoxide is collected and chemically changed using the solid material including copper as a catalyst or an adsorbent. However, since cuprous compounds are not easily dissolved even in a large amount of a solvent, cupric compounds are combined with the solid material, and then come into contact with a reductant, such as hydrogen or carbon monoxide, under suitable conditions, so that the cupric compounds are converted into cuprous ions (Cu+) or copper metal (Cu), thereby improving the selectivity for carbon monoxide.
In addition to this, since the cuprous compounds do not readily dissolve in most solvents, it is not easy to uniformly disperse a desired amount of cuprous compound in a solid material.
Methods of dispersing a cuprous compound in a solid material include a method of depositing an aqueous cupric salt on a solid material and then reducing the deposited cupric salt to a cuprous state, a method of physically mixing a solid support with a solid cuprous salt to form a mixture and then heat-treating the mixture at high temperature, a method of vaporizing a cuprous salt and then depositing the vaporized cuprous salt on the inner surface of a solid support, and other methods.
In this case, hydrogen, carbon monoxide, a tin compound, iron compounds, cobalt (II), dicarboxylate, reducing sugar, and the like are used in order to reduce the cupric salt to a cuprous state.
Hereinafter, the aforementioned methods of dispersing a cuprous compound in a solid material will be briefly described with reference to prior patents.
First, it was disclosed in U.S. Pat. No. 3,789,106 that a zeolite adsorbent or a mordenite adsorbent, which is prepared by exchanging sodium ions for copper ions or other metal ions, is used in order to adsorb carbon monoxide. The main object of such an adsorbent is to remove an infinitesimal amount of carbon monoxide present in a gas mixture. This patent is characterized in that the zeolite adsorbent or mordenite adsorbent is prepared by exchanging sodium ions for copper ions in a mordenite molecular sieve structure.
Further, U.S. Pat. No. 4,019,879 discloses a method of adsorbing and separating carbon monoxide (CO) from gas streams using an adsorbent formed of zeolitic molecular sieves exchanged with cuprous cations. In this patent, the adsorbent is prepared by impregnating zeolitic molecular sieves with cupric ions and then reducing the cupric ions to cuprous ions during an ion-exchange procedure.
Further, U.S. Pat. No. 4,470,829 discloses an adsorbent for selectively adsorbing carbon monoxide, comprising copper halide, aluminum halide, and polystyrene or its derivatives; and an adsorbent for selectively adsorbing carbon monoxide, comprising copper halide, aluminum halide, and activated carbon or graphite. Such an adsorbent is prepared by mixing the three components in a hydrocarbon solvent and then removing the hydrocarbon solvent.
Further, it was disclosed in U.S. Pat. No. 4,587,114 that an adsorbent for selectively adsorbing carbon monoxide can be prepared by impregnating a carbon support with a cuprous compound or a cupric compound and then removing the solvent therefrom. Here, the solvent is selected from among water, aqueous hydrochloric acid or ammonium formate, primary or secondary alcohols having 1 to 7 carbon atoms, acetone, ethyl acetate, formic acid, acetic acid, benzene, toluene, propionitrile, acetonitrile, aqueous ammonia, and the like.
Further, U.S. Pat. No. 4,713,090 discloses an adsorbent for selectively adsorbing carbon monoxide, comprising a composite support composed of silica and/or alumina and activated carbon, and a copper compound supported on the composite support. Here, the solvent, used in the preparation of the adsorbent, is selected from among aqueous ammoniacal formic acid, aqueous ammonia, nitrogen containing compounds, such as propionitrile, acetonitrile, diethylamine, dimethylformamide, N-methylpyrrolidone, etc.
Further, U.S. Pat. No. 4,914,076 discloses an adsorbent for selectively adsorbing carbon monoxide, prepared by impregnating an alumina or silica-alumina carrier with a cupric salt using a solvent containing a reducing agent and then removing the solvent. In this patent, the cupric salt is reduced to a cuprous salt, and water containing a reducing agent is used as the solvent in order to impregnate an alumina or silica-alumina carrier with a cupric salt. In addition, formalin, formic acid, alcohols and other compounds are used as the solvent. As the reducing agent, low atomic valence metal salts, such as iron compounds, tin compounds, titanium compounds, chromium compounds, and the like, aldehydes, saccharides, formic acid, oxalic acid, and other compounds are used.
Further, U.S. Pat. No. 4,917,711 discloses a method of preparing an adsorbent for selectively adsorbing carbon monoxide by mixing a support selected from among zeolite, alumina, silica gel, aluminosilicate, aluminophosphate and a combination thereof with a cuprous compound in a solid form or by mixing them with a solvent. Here, the solvent is selected from among water, an aqueous hydrochloric acid-containing solution, primary or secondary alcohols having 1 to 7 carbon atoms, acetone, ethyl acetate, hydrocarbons having 4 to 7 carbon atoms, propionitrile and acetonitrile, and the cupric compound, supported on the support, is converted into a cuprous compound.
Furthermore, U.S. Pat. Nos. 5,175,137, 5,126,310, and 5,258,571 disclose adsorbents for selectively adsorbing carbon monoxide by uniformly dispersing copper on a support using a dispersant, for example, ammonium citrate.
Meanwhile, in industrial fields, hydrogen is produced by reforming natural gas, naphtha and other hydrocarbon sources with steam or recovering hydrogen from various gases generated from petrochemical plants.
Raw materials, which are introduced into a hydrogen refining process necessarily performed at the time of producing hydrogen, include impurities, such as carbon dioxide, nitrogen, methane, water, carbon monoxide, and the like. Among the impurities, carbon monoxide is difficult to remove.
In order to remove the impurities, conventionally, activated carbon and zeolite 5A are used while using a pressure swing adsorption (PSA) process, and, particularly, the zeolite 5A is used as an adsorbent for selectively and reversibly adsorbing carbon monoxide.
However, the zeolite 5A adsorbent is problematic in that, since it has strong adsorptivity for carbon dioxide and water, its capacity to adsorb carbon monoxide is decreased when carbon dioxide and water are adsorbed on the zeolite 5A adsorbent, and thus the hydrogen products, produced using the zeolite 5A adsorbent through a pressure swing adsorption (PSA) process, cannot help including a large amount of carbon monoxide.
Therefore, in order to solve the problem, a pressure swing adsorption tower must be filled with an excessive amount of the zeolite 5A adsorbent.